What is the difference between synchronized and non-synchronized handovers?

In synchronized handover, the source and the target base stations (BTS) are synchronized to the same system clock. Here synchronization means that the timing of the TDMA frame at the BTSs is the same, i.e. the timeslot zeros from the BTS transmitted are synchronous with the timeslot zeros of the carriers on the set of neighbouring BTSs. However, the frame numbers need not be the same. All timings are to be referenced at the BTS.

So in synchronized handover, a mobile station (MS) is capable of obtaining uplink synchronization to the new cell prior to the cell access in the new cell. The MS is able to calculate the timing advance based on the TA in the source cell and the time difference between the signals received from the source and the target cells.

In a non-synchronized handover, the source and target cell system clocks are different. So the target cell sends "Physical Information" message to the MS with the new TA value.

Synchronized handovers can be used only in intra-BTS handover or inter-BTS handover when the BTSs are sectorized or collocated, in which case the equipment is located closely enough together to allow for fine synchronization in an inexpensive way.